Reference Guide

Chapter 3 47

HP-UX Architecture Conventions

Compiler Conventions

In order to write assembly language procedures that can both call to and

be called from high-level language procedures, it is necessary to

understand the standard procedure-calling convention and other

compiler conventions.

On many computer systems, each high-level language has its own calling

convention. Consequently, calls from one language to another are

sometimes difﬁcult to arrange, except through assembly code. The

architecture generally prescribes very few operations that must be done

to effect a procedure call, and there is often a pair of machine-language

instructions to call a procedure and return from one. PA-RISC

architecture provides no special procedure call or return instructions.

There is, however, a standard procedure-calling convention for all

high-level languages as well as the Assembler. It is tuned for the

architecture, and is designed to make a procedure call with as few

instructions as possible.

Besides deﬁning a uniform call and return sequence for all languages,

the calling convention is important for other reasons. In order to

streamline the calling sequence, the return link is not saved on the stack

unless necessary and the previous stack pointer is rarely saved on the

stack. Therefore, it is not usually possible to obtain a stack trace at an

arbitrary point in the program without some additional static

information about each procedure's stack frame size and usage.

For example, you could not obtain a stack trace while debugging or

analyzing a core dump, or using the TRY/RECOVER feature in HP

Pascal/HP-UX. Obtaining a stack trace is made possible by the stack

unwind mechanism. It uses special unwind descriptors that contain the

exact static information needed for each procedure. These descriptors are

generated automatically by the linker based on information provided by

all high-level compilers as well as the Assembler.

Each descriptor contains the starting and ending address of a

procedure's object code, plus that procedure's stack frame size, and a few

ﬂags indicating, among other things, whether the return link is saved on

the stack. Given the current program counter and stack pointer, the

stack unwind mechanism can determine the calling procedure by ﬁnding